Area Forecast Discussion
Issued by NWS Aberdeen, SD

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000
FXUS63 KABR 172322 AAA
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
622 PM CDT Mon Apr 17 2017

.SHORT TERM...(This Evening through Tuesday)
Issued at 251 PM CDT Mon Apr 17 2017

The forecast challenges are how much areal coverage of thunderstorms
will there be and how strong could they become tonight?

Currently, under an increasingly cloudy sky, temperatures are
warming through the 40s and 50s on generally east-southeast winds of
5 to 15 mph. Surface high pressure is exiting stage "east" this
afternoon and return flow breezy/windy southeasterly boundary layer
winds are starting to pick up some gust character as well. The
latest 850hpa dewpoint analysis shows +1C to +5C moisture advecting
northward on these stronger winds over western NE and western SoDak.

Fast forward to later this evening: Latest Rap Refresh model
guidance shows strong low level moisture advection in the low level
jet overnight, while strong mid/upper level jet streak winds
overspread the region. A strong mid level shortwave of low pressure
(still digging over the northern Rockies) will move across the
northern plains tonight. The latest Rap Refresh suggests lee-side
troffing will consolidate into a surface low between Rapid City and
Philip this evening before it moves northeast to near Aberdeen by
09Z Tuesday and Wheaton MN by 12Z Tuesday. Between the lift
associated with this low and the interaction of low level and upper
level jets and available low level moisture (for April), model progs
of instability and shear appear to be enough to support thunderstorm
activity tonight. Not much in the way of precipitation is expected
over this cwa prior to 00Z (7 PM CDT), but the 12Z/18Z suite of CAM
solutions today are now all generating thunderstorms that appear to
be routed in the boundary layer over UNR`s cwa (between 21Z and 03Z)
which they then lift northeastward into this cwa (becoming elevated)
mainly at or after 02Z (9 PM CDT) on the nose of the best low level
jet forcing and 850hpa moisture advection. In fact, several
solutions also hint at a separate secondary cluster of thunderstorms
developing in/north of the surface low further north and west near
the border of the Dakotas and Montana closer to the core of the
upper circulation/dynamic lift later after 04Z and sweep those
storms east-northeast through north central South Dakota before
lifting up into North Dakota. If convection does indeed develop, it
will need to be monitored mainly for potentially large hail later
this evening mainly west of a line from Mound City to Miller. East
of that line, sub-severe small hail could occur in the strongest of
any ongoing convective activity later in the night, along with brief
heavy downpours. Precipitation from this potential convective
activity should be clearing the far eastern forecast zones early
Tuesday morning. Presently, still looks like guidance is generating
between a quarter inch and a little over a half inch of
precipitation with this precip scenario. Breezy north-northwest
winds will be in play Tuesday morning mainly over northeast South
Dakota in the wake of this low pressure system until the pressure
gradient between the departing low and an approaching area of high
pressure relaxes. Tuesday should be a dry day, with high
temperatures running generally near to perhaps a degree or two above
normal.

.LONG TERM...(Tuesday Night through Monday)
Issued at 251 PM CDT Mon Apr 17 2017

West to southwesterly flow continues at 500mb at the start of the
period as the low off the Pacific Coast continues to eject waves
around and into the central U.S. The sfc low will be over the Upper
Great Lakes, with a sfc ridge extending from central Canada down
through the eastern Dakotas. This ridge will shift into MN by
Wednesday morning while the next sfc low moves along the western
NE/KS. While the main boundary with all the warm air and better
chance of convection stays in NE/KS, expect more of a steady rain to
move across our area. A widespread 0.25 to over 0.5 inches is
anticipated, although the location of highest potential will likely
shift as the time nears and forecast solutions become a little more
consistent. The initial 500mb wave differs in intensity/timing. A
secondary wave and sfc low across eastern MT late Wednesday
afternoon will likely be associated with some precipitation. There
is a small potential of for light precipitation extending from ND
across our northern counties at through the day Thursday and into
Thursday evening as a ridge builds in at 500mb. In our southern
counties, high pressure will be the rule, as a sfc high crosses NE
and extends a ridge up trough our area. The 500mb ridge will shift
to our east Friday, as the remnants of the Pacific low move into the
western half of the nation and organizes into a progressive low
across KS Friday night. A little more uncertainly comes into play
for the end of the week and into the weekend. We look to be in
between organizing sfc low pressure near OK and large high pressure
across much of western and central Canada on Saturday. The 500mb low
should eject northeast across the mid MS valley Saturday night, with
a more amplified ridge building across the Northern Plains. Yet
another system, this time in the form of an elongated sfc trough
looks to stretch from southern Saskatchewan through the western
Dakotas and western OK late Sunday night. With a more amplified
northern stream, will need to wait and see how much moisture is
available to get widespread showers across our area.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday Evening)
Issued at 620 PM CDT Mon Apr 17 2017

VFR conditions are expected for much of the taf period, except for
perhaps at KABR and KATY late tnt when some mvfr cigs may form.
Showers and thunderstorms will march across the state overnight,
brief ifr conditions can be expected with any storms. The wind
will be gusty out of the southeast through much of the night.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
&&

$$

SHORT TERM...Dorn
LONG TERM...KF
AVIATION...TDK



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